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The human body is a complex system of pathways that allow communication between the brain and the spinal cord. The corticospinal tract is one of the most important pathways in the central nervous system, carrying movement-related information from the brain to the spinal cord. The rubrospinal tract is another important pathway that originates in the midbrain and sends corrective commands to the spinal cord. Descending tracts are responsible for motor actions, carrying signals from the brain to lower motor neurons, which then directly innervate muscles to produce movement. These tracts can be divided into pyramidal tracts, which originate in the motor cortex, and extrapyramidal tracts, which originate in the brain stem.
| Characteristics | Values |
|---|---|
| Tract that sends muscle messages | Corticospinal tract |
| What it does | Carries movement-related information from the brain to the spinal cord |
| Number of nerve fibres | Approximately 1 million |
| Average conduction velocity | 60m/s |
| What the nerve fibres are covered with | Myelin sheaths |
| Where the fibres come from | Supplementary motor area (SMA), premotor cortex (PMA), parts of the somatosensory areas (S1 and S2) and parts of the posterior parietal cortex |
| What the fibres stimulate | Activity in different types of muscles |
| Types | Lateral corticospinal tract and anterior corticospinal tract |
| What the lateral corticospinal tract does | Controls the movement of muscles in the limbs |
| What the anterior corticospinal tract does | Involved with movement of the muscles of the trunk, neck, and shoulders |
| Where the fibres terminate | Thoracic, lumbosacral and cervical levels |
| What the extrapyramidal tracts do | Carry motor fibres to the spinal cord |
| What the pyramidal tracts do | Carry motor fibres to the spinal cord and brainstem |
| What the rubrospinal tract does | Augments the activity of the flexor muscles and inhibits the action of the extensor (antigravity) muscles |
| What the reticulospinal tract does | Regulates the function of spinal reflex arcs and maintains muscle tone when standing and walking |
| What the vestibulospinal tract does | Controls body balance |
| What the tectospinal tract does | Responsible for the blinking reflex and eye pursuit movements when following an object |
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What You'll Learn
The corticospinal tract
At the base of the pyramids, the corticospinal tract divides into two: the lateral corticospinal tract and the anterior (or ventral) corticospinal tract. The lateral corticospinal tract forms about 90% of the connections in the corticospinal tract, with most of the fibres crossing over to the other side of the brainstem (decussating) in the medulla. These fibres then descend into the spinal cord, terminating in the ventral horn, from where the lower motor neurons supply the muscles of the body. The lateral corticospinal tract primarily controls the movement of muscles in the limbs.
The anterior corticospinal tract forms the remaining 10% of the corticospinal tract fibres. These fibres stay on the same side (ipsilateral) in the spinal cord but then cross over at the level of the spinal nerve in which they exit. The anterior corticospinal tract is involved with the movement of the muscles of the trunk, neck, and shoulders.
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The rubrospinal tract
The tract begins as axons of the neurons in the red nucleus and descends through the white matter of the pons and medulla oblongata. It then enters the lateral white column of the spinal cord, terminating and synapsing with interneurons at different levels. Each rubrospinal fiber terminates in a specific area, primarily in the cervical and upper cervical segments of the spinal cord, indicating its role in upper limb control rather than lower limb control.
While the rubrospinal tract is small and rudimentary in humans, it is believed to have a more significant role in other primates. Experiments have shown that in some primates, the rubrospinal tract can take on the functions of the corticospinal tract when the latter is lesioned. This suggests that the rubrospinal tract may have the capacity to assume a more substantial role in muscle control under certain conditions.
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The pyramidal tracts
The corticobulbar tract conducts impulses from the brain to the cranial nerves. These nerves control the muscles of the face and neck and are involved in facial expression, mastication, swallowing, and other motor functions. The corticospinal tract conducts impulses from the brain to the spinal cord. It is made up of a lateral and anterior tract. The corticospinal tract is involved in voluntary movement. The fibres within the lateral corticospinal tract decussate (cross over to the other side of the CNS). They then descend into the spinal cord, terminating in the ventral horn (at all segmental levels). From the ventral horn, the lower motor neurons go on to supply the muscles of the body. The anterior corticospinal tract remains ipsilateral, descending into the spinal cord. They then decussate and terminate in the ventral horn of the cervical and upper thoracic segmental levels.
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The extrapyramidal tracts
There are four extrapyramidal tracts in total. The vestibulospinal and reticulospinal tracts do not decussate, providing ipsilateral innervation. The rubrospinal and tectospinal tracts do decussate, providing contralateral innervation. The reticulospinal tract is one of the most important extrapyramidal tracts for controlling the activity of lower motor neurons.
Extrapyramidal tracts are clinically important in distinguishing between the effects of damage involving the basal ganglia and those of damage to the classic "pyramidal" pathway. Damage to the pyramidal tracts was previously thought to be the sole cause of epileptic motor activity, but it was discovered that extrapyramidal tracts also play a role in delivering such activity. Extrapyramidal tract lesions are commonly seen in neurodegenerative diseases, encephalitis and tumours, resulting in various types of dyskinesias or disorders of involuntary movement.
The extrapyramidal system (EPS) is polysynaptic in nature and is composed of several tracts and nuclei. The tracts include the reticulospinal, vestibulospinal, rubrospinal, and tectospinal tracts. The EPS provides control of reflexes that accompany responses to affective and attentive situations, as well as control of movements that are originally voluntary. It is essential in maintaining posture and regulating involuntary motor functions.
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The reticulospinal tract
The medial reticulospinal tract descends ipsilaterally in the anterior funiculus. It is responsible for controlling axial and extensor motor neurons, enabling extension of the legs to maintain postural support. Stimulation of the midbrain locomotor centre can result in patterned movements, such as stepping. The fibres of this tract arise from the caudal pontine reticular nucleus and the oral pontine reticular nucleus and project to lamina VII and lamina VIII of the spinal cord.
The lateral reticulospinal tract descends bilaterally in the lateral funiculus. It is responsible for inhibiting the medial reticulospinal tract and, therefore, extensor motor neurons, enabling the modulation of the stretch reflex. The tract inhibits voluntary movements and reduces muscle tone. The fibres of this tract arise from the medullary reticular formation, mostly from the gigantocellular nucleus, and descend the length of the spinal cord in the anterior part of the lateral white column (funiculus). The tract terminates in lamina VII, mostly with some fibres terminating in lamina IX of the spinal cord.
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Frequently asked questions
The corticospinal tract is the largest descending tract in humans and is responsible for carrying movement-related information from the brain to the spinal cord.
The tracts can be functionally divided into two major groups: pyramidal tracts and extrapyramidal tracts.
Pyramidal tracts originate in the motor cortex and carry motor fibres to the spinal cord and brainstem. They are responsible for the voluntary control of the muscles of the body and face.
Extrapyramidal tracts originate in the brain stem and carry motor fibres to the spinal cord. They are responsible for the involuntary and automatic control of all muscles, including muscle tone, balance, posture and locomotion.
